Tripping circuit for static switches

ABSTRACT

A D.C. trip circuit is provided for power system wiring protection during faults and overloads, which circuit closely approximates the characteristics of previously used thermal circuit breakers, particularly the trip time. Solid state components are used to develop a signal whose magnitude depends on simple circuit functions yet is effective to produce a trip signal wherever its magnitude is equal to or greater than zero. The trip time is close to that resulting from an I2T (load current squared times trip time) value while requiring simpler, and hence more reliable, components to determine.

United States Patent Fox 4 1 Oct. 10,1972

TRIPPING CIRCUIT FOR STATIC SWITCHES David A. Fox, Lima, OhioWestinghouse Electric Corporation, Pittsburgh, Pa.

Filed: March 15, 1971 Appl. No.: 124,309

Inventor:

Assignee:

US. Cl ..317/36 TD, 317/141 S, 317/148 SR Int. Cl. ..HOlh 47/18 Field ofSearch ..317/36 TD, 141 S, 33, 142,

References Cited UNITED STATES PATENTS 4/1971 Lipnitz ..317/36TD 3/1969Zocholl ..3l7/36TDX l6 REFERENCE SIGNAL TO ITOAD ,m 0c SHUNT PrimaryExaminerGera1d Goldberg Attorney-A. T. Stratton, F. P. Lyle and GordonH. Telfer [5 7] ABSTRACT 3 Claims, 3 Drawing Figures REFERENCE SIGNALwere T0 CONTROL OR CT CONVERTOR LOAD CU R RENT 22 TRIP ACE SIGNAL 4MAMPLIFIERJ PATENTEDnm 10 1972 SHEET 2 OF 2 P] I. w .5616 SE28 v p 0m HF5050 92: 19:26 5301 225 M\ Q. J F n uomDom mosh O TRIPPING CIRCUIT FORSTATIC SWITCHES BACKGROUND OF THE INVENTION 1. Field of the InventionThe invention relates to electronic circuitry and particularly to solidstate (static) switches and associated circuitry.

2. State of the Art There has been recent interest in replacing thermalcircuit breakers with static switching elements for protective devicesin electrical power systems. Particular emphasis in this direction hasbeen in the field of aircraft electrical systems where the higherreliability and other advantages of static elements are particularlydesirable. For background, reference may be made to copendingapplication Ser. No. 92,348 filed Nov. 24, 1970 by D. E. Baker andassigned to the assignee of the present invention. The copendingapplication is representative of a type of circuit that does not haveinherent current limiting capability, that is, it is operable onlybetween fully on and off states.

One of the desirable functions of either static or electromechanicalcircuit breakers is to protect the wiring in the system during faultsand overloads. Wire current handling capability not only depends on thecurrent magnitude but also the time period such current is present. Thiscapability is often expressed as an I T value where I is the current inthe load circuit and T is the trip time. For example, for an FT value of4000 amperes -seconds which is typical for No. 22 wire, any givencurrent (tag, 20 amperes) larger than the steady state rating of thewire (5.5 amperes for No. 22 wire) must be stopped before a fixed time(10 seconds) elapses. I

The PT factor is particularly important in aircraft power systems wherethere is great emphasis in using the lightest wire possible. Of courseit is also very desirable not to shut off a load circuit prematurely orunnecessarily. Therefore a means that is responsive to the PT value,when it reaches the limit imposed by the wire, is necessary. In thepast,'thermal circuit breakers could be selected that closelyapproximated the PT capability of the wire. For static switches it isnecessary to provide some means that electronically performs thisfunction.

If a straightforward electronic calculator with some form of multiplierwere made to calculate the time IT and give a trip signal before the PTvalue of the wire is reached, it would be fairly complex and hencerelatively expensive to achieve necessary reliability. The PT valuereferred to is actually the integral of I'dt. Thus it would appearnecessary to use a multiplier or squaring circuit to produce the productof I times I and then an integrator to produce the integral of thatproduct over time, t.

Various types of electronic time delay circuits are of course known andused in the art of electronic circuitry for various protectivefunctions. However, such protective schemes do not as closely match theactual power handling capabilities of the system, that is, they must bebuilt with a wide safety margin under certain operating conditions toprovide a minimal safety margin under others.

SUMMARY In accordance with this invention an electronic circuit withsolid state components is provided that in a relatively simplearrangement of arithmetic stages and an integrator, without requiring amultiplier, provides a reliable and effectively useful approximation ofPT in a current carrying conductor.

DRAWINGS FIG. 1 is a schematic diagram, in functional block form, of anembodiment of the present invention;

FIG. 2 is a schematic diagram of a more specific example of the presentinvention; and

FIG. 3 is a set of curves illustrating the performance of a circuit inaccordance with this invention compared with the performance of priorart apparatus.

PREFERRED EMBODIMENTS Referring to FIG. 1, the general nature ofapparatus in accordance with this invention is shown. A means 10 isprovided to develop an electrical signal X that is proportional to theload current. The description herein will primarily refer to theoperative signals as voltages although a description could also be madein terms of another parameter, such as current. The means 10 includes asignal source 12 that develops a signal from the load current. Thesignal source -12 is suitable for the type of power in the load circuit.For example, a resistive shunt may be used in a D.C. circuit or acurrent transformer (CT) may be used in an A.C. circuit. The

signal from the source 12 is processed by means 14 such as an amplifier,in the case of D.C., or some form of A.C. to DC. converter, to produce avoltage, X. A reference voltage source 16 supplies a direct voltage C ofopposite polarity to X that is subtracted from voltage X at a summingpoint 18 to produce a new signal (X-C). During the time, T, that thisdifierence signal is positive, where X is positive, it is integrated inan integrator 20 that has a gain arbitrarily designated as l/A seconds.The integrator output, 1/A (XC)T, is summed at a summing point 22 withthe original quantity X and also with a voltage B, of opposite polarityto X, from reference voltage source 24. The resultant signal is suppliedto a trip signal amplifier 26 that is responsive to any zero or positivesignal to generate a trip signal, that is, when 1/A (X-C)T+X- B 2 0.

From the above, it can be seen that a trip signal is produced when,

where T is the time required to produce a trip signal upon occurrence ofcertain load current;

X, B, and C are voltages at the points indicated in the schematic ofFIG. 1; and

A is the inverse of the gain of integrator 20.

Ideally, 7' should equal DIX, where D is a constant proportional to thedesired FT limit. The signal obtained by the FIG. 1 circuit can be shownto closely approximate that condition where X is of a magnitude largerthan C, and smaller than B.

Referring to FIG. 2, a more detailed schematic diagram of a circuit isshown merely by way of example within the general type shown in FIG. 1.Portions of FIG. 2 are identified by the same reference numerals ascorresponding elements of FIG. 1 where appropriate. The FIG. 2 circuitis a D.C. trip circuit that has a means for developing a current intopoint 18 that has a first component from line 31 that is proportional tothe load current.

The voltage applied from line 31 is that referred to as X in thedescription of FIG. 1. For circuit convenience, X is actually a negativevoltage with respect to line 70.

The means 10 includes a DC. shunt 12 on supply line 32 between D.C.supply 71 and static power switch 72. The DC. shunt 12 supplies acurrent dependent signal to operational amplifier stages 14A and 148that respectively shift the level and amplify the shunt signal,producing the desired signal, X.

At a summing point 18 signal X is algebrically added to a positivereference current, developed by a large resistor 34 fromthe supply 32(corresponding to means 16 of FIG. 1 This positive current correspondsto reference voltage C referred to in connection with FIG. 1 from source16. Thus the current into point 18 from 31 and 34 can be referred to asX-C. Actually two currents developed through respective resistors 33 and34 are added at point 18 to develop an (X-C) signal rather than voltagesper se.

Operational amplifier stage 20 serves to sum the current signals into 18and also functions as a clamped integrator to produce at its output asignal corresponding to l/A (XC)Twhere l/A is the gain of stage 20 andTis the time that X is greater than C. Diodes 39 and 40, connected asshown, are to prevent the output of the integrator stage 20 from goingtoo far negative. Resistors 35 and 54 and diodes 36 and 37 prevent thesignal from going negative.

If the resistors 54 and 35 are properly selected the current throughdiodes 36 and 37 will be equal and the voltage at point 73 will beexactly zero unless the signal, X, exceeds the reference, C.

The signal, X, on line 74, is further combined with a signalcorresponding to reference voltage B referred to in connection with FIG.1 from source 24. The voltage at point 75, derived from the currentsthrough resistors 43 and 56 can be referred to as B-X.

A final operational amplifier stage 26 has as inputs the integratoroutput, I/A(X-C)T and the quantity B-X. The several inputs are comparedby Operational amplifier element 26 to produce a trip signal whenl/A(X-CB The trip signal produced by element 26 is supplied, usuallythrough an intermediate on-off control circuit 80, to the power switch72 to turn off the switch, and thus deenergize a load 73.

The dashed line box identified by the reference nuemral 85 in FIG. 2encloses those elements of the circuit that functionally correspond tothe summing point 22 and amplifier 26 of FIG. 1, which are also enclosedby a box 85. The summing function in FIG. 2 is provided partly outsideamplifier 26 to develop a (B-X) signal at point 75 and partly withinoperational amplifier 26 having point 75 connected to the positiveamplifier input terminal while the integrated signal from point 73 issupplied to the negative amplifier input terminal.

The circuit of FIG. 2 is merely exemplary and subject to considerablemodification using known components and circuit design techniques. Theembodiment of FIG. 2 is presently preferred because it requires only areasonable number of readily available components which can be formedwith other control circuitry on a single hybrid substrate to formapparatus suitable as each of the remote power controllers inapplications such as aircraft power systems.

The following is a more comprehensive enumeration of the components tobe connected as shown in the exemplary circuit of FIG. 2 giving suitablecomponent values or other identification.

Operational amplifiers 14A, 14B 20, and 26 Type 741 (Fairchild 11. A741or equivalent high 2 performance opera tional amplifier) IN9I4 12v.breakdown Diodes 36, 37, 38, 39, and 40 Zener diode 41 Power source 60+28v. D.C. nominal The resistor 48, diode 38, zener diode 41, and PNPtransistor 59 provide a power supply isolated from ground for theoperational amplifiers and associated elements and allow operation overa range of, in this example, from ll volts to volts for the DC. supplyvoltage without adverse affect on performance.

Circuits as described in the foregoing example have been made andsuccessfully operated. FIG. 3 illustrates the results with current(expressed as a multiple of steady state current rating) plotted againsttrip time, in seconds, on alog-log scale. Curve A is for the describedcircuit of FIG. 2. Curve B is a line plotted for PT 10(Per UnitamperesP-seconds where 1 Per Unit Ampere (or 1 P.U. amp.) is percent ofrated current. The value of 10 for PI is an arbitrarily selected valueselected as a protection level for a given wire. Curves A and B showclose conformance over a considerable range, from about I P.U. to about10 P.U. amperes which is a desirable range for protection of aircraftwiring. For comparison, curves C and D are shown to indicate theperformance limits of typical thermal circuit breakers over thetemperature range encountered in aircraft. Curve E is a typicalcharacteristic for an actual Wire (e.g. No. 22 wire); this handbookcharacteristic is determined by the size of wire and the nature of itsinsulation and represents the level at which actual damage can beexpected.

In contrast to thermal circuit beakers, circuits in accordance with thisinvention are essentially unaffected by temperature changes orvibration. If high quality (low temperature drift) components are used,the circuit will perform within percent of the characteristicillustrated by curve A for all temperatures within the above-mentionedrange.

Certain types of static switch circuits would not require a trippingcircuit as disclosed here. For example, the DC static switch circuits ofcopending applications Ser. Nos. 124,310 and 124,232, both filed Mar.15, 1971 by D. E. Baker and assigned to the present assignee, have.current limiting capability that makes the trip circuit unnecessary.However, there are a number of other types of static switches that needa trip circuit, particularly those in AC. load circuits.

1 claim:

1. Electronic apparatus for producing a signal indicating a current on aconductor has endured for a time endangering the conductors safetycomprising: means to develop a first signal having a parameter ofmagnitude proportional to the magnitude of current in the conductor;means to sum said first signal with a reference second signal ofopposite sign to said first signal and of smaller magnitude to develop adifference signal; means to integrate said difference signal with timeto develop an integrated signal; and means to sum (1) a reference thirdsignal of opposite sign to said first signal and of larger magnitude,(2) said integrated signal, and (3) said first signal.

2. The subject matter of claim 1 further comprising: means responsive tothe algebraic sum of voltages (l), 2) and (3) to produce a trip signalwhenever the sum is zero or of the same sign as said first voltage.

3. The subject matter of claim 2 wherein: said named means compriseactive electronic circuit elements all of which are solid state.

* l lit t

1. Electronic apparatus for producing a signal indicating a current on aconductor has endured for a time endangering the conductor''s safetycomprising: meAns to develop a first signal having a parameter ofmagnitude proportional to the magnitude of current in the conductor;means to sum said first signal with a reference second signal ofopposite sign to said first signal and of smaller magnitude to develop adifference signal; means to integrate said difference signal with timeto develop an integrated signal; and means to sum (1) a reference thirdsignal of opposite sign to said first signal and of larger magnitude,(2) said integrated signal, and (3) said first signal.
 2. The subjectmatter of claim 1 further comprising: means responsive to the algebraicsum of voltages (1), (2) and (3) to produce a trip signal whenever thesum is zero or of the same sign as said first voltage.
 3. The subjectmatter of claim 2 wherein: said named means comprise active electroniccircuit elements all of which are solid state.